Permeable membrane process and systems have been increasingly employed in gas separation operations. In such operations, a gas mixture is fed into the nonpermeate side of at least one gas separation. membrane module under pressure. Upon contacting the surface of the membrane in the nonpermeate side of at least one gas separation membrane module, a more readily permeable component of the gas mixture passes or permeates through the membrane. The resulting permeated gas component is recovered or removed from the permeate side of the gas separation membrane module while the remaining unpermeated gas component is removed or recovered from the nonpermeate side of the gas separation membrane module. Commonly, these gas separation processes and systems are started up by passing and venting a compressed gas stream into and from the nonpermeate side of at least one gas separation membrane module until the desired steady state operating pressure, temperature and product purity are reached.
The gas mixture fed to the gas separation membrane module, however, may contain a substantial amount of moisture and may cause corrosion and condensation in instrumentation, piping, pneumatic tools, ventilators and other equipment associated with the gas separation membrane module. In certain instances, it may also lead to inferior performance of the gas separation membrane module and/or other equipement, such as adsorption traps. To prevent such problems, a membrane dryer module is employed to provide a dry gas to the gas separation membrane module. Many of these arrangements use waste permeate gas stream from the permeate side of the gas separation membrane module to purge the permeate side of the membrane dryer module during steady state operations. European patent application 0430304, U.S. Pat. No. 5,169,412 and U.S. Pat. No. 4,931,070, for example, describe steady state operation of an air separation system with a membrane dryer module.
Conventional membrane dryer module start-up methods include feeding a compressed gas into the nonpermeate side of a membrane dryer module and using at least a portion of the resulting product from the non-permeate side to purge the permeate side of the membrane dryer module. U.S. Pat. 5,131,929 describes a startup method for a membrane drying module in which the inlet to the dryer module is brought to a specified minimum pressure prior to allowing gas to flow into the nonpermeate side of the dryer module. The dryer module is then operated dead ended until the pressure in the dryer module reaches the operating pressure (higher than the minimum inlet pressure). During this time, all of the gas is permeated or passed to the permeate side of the dryer module. This allows the dryer module to be started up and reach the desired dew point.
The most common method of membrane dryer shutdown is to turn off the gas supply and let the pressure decrease to atmospheric. To efficiently operate the membrane dryer module in cyclical or intermittent service, U.S. Pat. No. 5,030,251 discusses using dry product backflow on the nonpermeate side of the dryer module either under pressure or at low pressure to dry out the moisture front in the membrane during shutdown. This method uses product from the nonpermeate side of the dryer module to back purge the nonpermeate side of the same to dehydrate the membrane.
None of these methods, however, is directed to improving the startup and shut down processes for a gas separation system with a membrane dryer module.
It is an object of the invention to provide improved startup and shut down processes for a membrane system comprising at least one membrane dryer module and at least one gas separation membrane module.
It is another object of the invention to provide membrane systems which are useful for carrying out the improved startup and shutdown processes.